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Background and Importance Extracorporeal membrane oxygenation (ECMO) may impact the pharmacokinetics of β-lactams and dose adjustments might be necessary to achieve therapeutic target attainment (TA).Aim and Objectives To assess variables influencing meropenem pharmacokinetics and TA in critically ill patients undergoing ECMO.Material and Methods Adult ICU patients (>18 years) at a tertiary hospital receiving meropenem 2g q8h(3h-infusion) under ECMO were prospectively enrolled (May 2022-August 2025). Trough serum concentrations (Cmin) at steady-state were measured using High-Performance Liquid Chromatography-Ultraviolet. TA was defined as Cmin>16 mg/L (4x minimal inhibitory concentration (MIC) for Acinetobacter spp, according to EUCAST breakpoints). Results are reported as n(%) for categorical variables and median (IQR) for continuous variables. Statistical analyses were conducted with Jamovi software using t-test or Mann–Whitney U, and with chi-square or Fisher’s exact test.Results A total of 23 patients (56.5% male) were included. Eight patients (34.8%) achieved TA, and within this group, 62.5% were female. For analysis, patients were classified according to target attainment: TA group(Cmin 32.3[26.1–51.8]) and non-TA group(Cmin 6.7[3.9–8.3]).No significant differences were found between TA and non-TA groups in sex (37.5% vs 66.7% male), age (55.7[53.1-62.7] vs 46.0[37.2-57.8] years), BMI (29.0[25.6-29.0] vs (26.9[23.6-30.5]), ECMO type (75% vs 93,3% veno-venous), or sampling time relative to meropenem initiation and circuit change. Clinical factors (obesity, mechanical ventilation, vasopressor use) showed no impact on TA, with similar outcomes in ECMO and 30-day mortality. Significant differences in serum creatinine (1.24 mg/dL TA vs 0.55 mg/dL non-TA; p=0.003) were observed between groups. Glomerular filtration rate(GFR) was estimated using CKD-EPI, Cockcroft-Gault, and MDRD-6 equations. CKD-EPI showed the strongest, though weak,inverse correlation with Cmin (r = –0.390), suggesting it only partially explained the variability. No patients with GFR>130 mL/min achieved TA, likely due to an augmented renal clearance. Diuresis was approximately twice as high in the non-TA group compared to the TA group (3186 mL/day vs 1657 mL/day; p=0.01). All patients undergoing continuous renal replacement therapy achieved TA.Conclusion and Relevance Results show that TA achievement is mainly influenced by renal function parameters, with higher GFR and increased diuresis associated with non-TA. These findings suggest that many patients may be underdosed, highlighting the need for further research on optimal dosing strategies in ECMO patients.Conflict of Interest No conflict of interest